The present invention relates to an optical wavelength multiplexing transmission of an optical fiber communication, and particularly to a constitution of an optical add/drop multiplexer used for an optical add/drop multiplexing section for carrying out communication between a plurality of nodes, a node constitution having an optical add/drop constitution, and an optical network.
In a long distance optical communication making use of an optical fiber, the transmission capacity by a single optical fiber was expanded rapidly by introduction of an optical wavelength multiplexing technique for placing independent signals on a plurality of different wavelengths in a single optical fiber. For example, by combining an optical fiber amplifier making use of the 1.5 xcexcm band optical amplification characteristics of erbium doped fiber with the wavelength multiplexing technique, it becomes possible to place the rate of 10 Gbit/s on each wavelength to transmit 160 wavelengths for several hundreds km.
Further, recently, there has been introduced the optical add/drop multiplexing (separation and multiplexing) technique in which in the optical signal transmission between a plurality of spots, a common optical fiber transmission path is used to vary spots of traffic every wavelength of multiplexed signals, whereby optical signals are relayed and connected directly without converting them into electrical signals. The wavelength multiplexing technique and the signal optical add/drop multiplexing are combined whereby the communication between specific two spot nodes can be carried out every wavelength making use of the common optical fiber, thereby realizing the inexpensive optical transmission between many spots.
For demultiplexing from a multiplexed signal, roughly two methods have been employed. Generally, there are a method for separating all wavelengths, and a method for a single channel drop which takes out only a specific wavelength by a wavelength filter. For example, in the center node, electrical signals are taken out of all the optical signals and processed, and therefore, the former method which separates all the wavelengths is employed. However, in the local node which requires to separate only a few wavelength optical signals out of optical signals subjected to scores of or hundreds of wavelength multiplexing, it is not economical that all the wavelength signals are separated and multiplexed, and in a case of passing through a plurality of nodes, the shaping effect of a wavelength filter occurs. The shaping effect termed herein is a band narrowing effect in which even lowering of loss of 0.3 dB in the shape of a single filter, ten times of transmission result in loss of 0.3 dB, and the 3 dB band in total corresponds to a 0.3 dB band of a single filter. Since in the latter method, the constitution having the single drop combined is economical and the transmission wavelength signal is not taken out by a wavelength filter, there is no influence of the shaping effect of a wavelength filter; therefore the constitution was used at a spot for demultiplexing/multiplexing only a few wavelength optical signals.
For a single channel drop section, there is a method making use of a dielectric multilayer film filter comprising a construction having a plurality of dielectric layers having a great refractive index difference laminated and an optical fiber bragg grating filter comprising a construction in which an optical signal for transmitting an optical fiber delicately changes in refractive index. Light is reciprocal, and an optical channel of the single channel drop section is reversed to thereby constitute the single channel add section.
In the constitution of the wavelength add/drop making use of the dielectric multilayer film filter, a single channel drop filter is connected to an optical fiber subjected to wavelength multiplexing, only specific wavelength is separated and taken out, an optical signal of the other wavelength is let to pass through as it is to reach the single channel add filter of the same wavelength, and signal light of the same wavelength as the drop wavelength is multiplexed. The dielectric multilayer film filter performs demultiplexing and multiplexing with a thickness of a few wavelengths, and the filter constitution is a multilayer film so that excellent mass production is presented, but the rejection characteristics of demultiplexed/multiplexed light to the transmission path is not satisfactory in principle, which is about dozens of dB. The leakage light to the transmission path suffers several tens dB of loss in a subsequent filter, which is in total about 30 dB, producing a coherent cross of the demultiplexed/multiplexed signal and the optical signal. According to General Meeting SB-11-7 (p. 747) of The Institute of Electronics, Information and Communication Engineers in 1996, the coherent cross talk requires rejection of about 38 dB or more. Normally, in a system using the single wavelength add/drop multiplexing, there are many ring constitutions via a plurality of nodes or long distance transmissions via a plurality of nodes, accumulation of grain differences between wavelengths by repeating of relay becomes large. Referring to specific wavelength signals, as the leakage characteristics of the demultiplexing section, a light level is sometimes higher than an average determined value, and since the multiplexed optical level has an optical level independently thereof, the rejection characteristics as high as grain differences are further required to prevent the coherent cross talk. Conventionally, to improve the rejection characteristics, there is employed a method for inserting a further filter between the drop filter and the add filter to enhance the rejection characteristics, but an increase in pass-through loss of transmission light or an increase in cost are brought forth.
In the constitution of the wavelength add/drop multiplexing making use of a fiber bragg grating filter, the fiber bragg grating is a reflection reversing type filter in which light reversibly moves in the same optical fiber as the moving-in channel as described in General Meeting SB-11-7 (p. 747) of The Institute of Electronics, Information and Communication Engineers in 1996, and therefore, and it was necessary to make use of a circulator for taking out the demultiplexed light or multiplexing insert light. In the circulator, three input positions and output positions can be separated according to the light traveling direction, and thereby the reverse-traveling light can be output not to the incident fiber but the demultiplexed light taking out fiber. The fiber bragg grating is excellent in wavelength selectivity, and leakage of demultiplexed light to the transmission channel is rejected not less than 40 dB. However, the manufacturing process of the fiber bragg grating is easy and the cost is low, but a circulator which is complex in construction and has less effect of mass production is required, and so, the price is high to obtain the add/drop multiplexing function. Further, recently, higher modulation speed of an optical signal and higher density wavelength multiplexing are progressed, and the constitution of a system for multiplexing the rate of 10 Gbit/s at 100 GHz intervals or 50 GHz intervals has been studied.
We have obtained from our studies that in the fiber bragg grating, a great program as noted below occurs. When a spacing between wavelengths is narrow, a filter band of the fiber bragg grating becomes narrow, and it is necessary to make small a change in refractive index forming a grating, for example, it is necessary to lower it to about 5xc3x9710 for 100 GHz spacing. To obtaine high rejection characteristics in that state, it is necessary to have a longer reflection area because a reflection coefficient in an area lowers by a portion that the change in refractive index lowers, for example, it is necessary to have not less than 10 mm in the above-described example. However, in a case where a digital signal of 10 Gbit/s is input, a spatial dispersion occurs due to the reflection from the incident end of the grating and the reflection from a deep point of a reflection area. For example, when the length of the practical fiber bragg grating is 10 mm, a delay caused by the spatial dispersion of 60 ps occurs, thus posing a problem that also as a NRZ signal, waveform deterioration is brought forth with respect to the pulse width of 100 ps.
Objects to be achieved by the present invention are to prevent a coherent cross talk, which is required in an optical add/drop multiplexer, and to avoid a spatial dispersion at the time of add/drop multiplexing of a high speed modulation signal. Further the above objects are to be realized while making a loss of a pass-through signal low and reducing costs.
To suppress wavelength deterioration resulting from demultiplexing/multiplexing of a high speed modulation signal in an optical wavelength multiplexing system whose wavelength spacing is narrow and occurrence of a coherent cross talk with a low cost, a filter for demultiplexing/multiplexing a specific wavelength from a multiplexed optical signal is used, a filter comprising a dielectric multilayer film is used for a filter for multiplexing a specific wavelength to the other transmission optical signal is used, and a fiber bragg grating filter is used as a rejection filter for rejecting a leakage of demultiplexed optical signal into a transmission path.
A dielectric multilayer film filter is short, a few xcexcm, in a multiplexing reflective area, and can be demultiplexed or multiplexed without bringing forth deterioration of waveform. Further, in the present constitution, by combining with a dielectric multilayer film filter, an optical circulator need not be used, and the high rejection characteristics of 40 dB is obtained with respect to rejection light signal by making use of the fact that the cost of the fiber bragg grating itself is low and that the loss is low with respect to transmitted light.
Since transmission rejection signal light is returned in the form of reflection reversal from the fiber bragg grating used for rejection, it is feared that the returning light is leaked into the demultiplexing end. Normally, the transmission rejection signal is passed through the dielectric multilayer film filter twice to damp not less than 20 dB, and reflection from a connector of the input fiber is not more than 20 dB, thus posing a problem. Otherwise, an optical isolator is inserted to reject the reflection reversing light from the fiber bragg grating filter, thereby removing the leakage light.
In general, for a filter for demultiplexing a specific wavelength from a multiplexed optical signal, when an optical signal having a specific wavelength demultiplexed to the other passing-through optical signal remains, a coherent cross talk occurs when the constitution for multiplexing an optical signal having a specific wavelength continues later, but with the present constitution, the coherent cross talk can be avoided by powerfully rejecting the leaking light having a specific wavelength.
By suppressing the wavelength deterioration of the demultiplexed light and the multiplexed light to suppress a coherent cross talk of the demultiplexed light and the multiplexed light low, and using an optical signal add/drop multiplexer making a loss of transmission light low, an optical network of high quality of signal and of low cost can be realized.